Field of Invention
The present invention relates generally to a power over Ethernet system, and more particularly to an inspection device and method for power devices in a power over Ethernet system.
Related Art
Power over Ethernet (PoE) has found widespread application in many areas. For example, IEEE has released two standards that relate to PoE—IEEE 802.3af in 2003 and IEEE 802.3at in 2009. These standards are adopted by many applications. In this disclosure, these relevant standards relevant to the PoE will be referred to as the “PoE standard” hereinafter. PoE technology allows supplying electric power through Ethernet to devices such as Internet phone, wireless stations, network cameras, hubs, and even computers without the need of extra power outlet. Combining data transmission and power supply, PoE technology can reduce the cost and complexity of the overall network computing system.
In a power over Ethernet (PoE) system, the electric power is provided by the power source equipment (PSE) via Ethernet data cable to the powered device (PD). The power source equipment may include an Ethernet switch, a router, or other network switching and midspan devices. In the PoE system, a power device is connected to the network and configured to obtain or request power from the power source equipment over the network.
In a PoE system, the power source equipment may connect to multiple power devices through their network connecting ports, but may also connect to devices that will not or cannot obtain power from the power source equipment. In a real application, the powered device may include devices that are in compliance with the PoE Standard, and devices that are compatible with the PoE Standard. The PoE Standard provides that, before a power source equipment may provide power to a powered device, the power source equipment must determine whether the particular powered device is in accordance with the PoE standard. In addition, most of the power source equipment, or the power supply equipment that incorporates power source equipment, will also determine whether a device is compatible with the PoE standard. This may include a legacy device that is compatible with the PoE Standard. If compatible, the power source equipment will also provide power to such compatible device.
Under the PoE standard, when carrying out said inspection, the power source equipment provides a signal to the connecting port of a target device, and detects the response signal from the connecting port. If the response signal indicates a signature resistance ranging from 19k to 26.5k ohms, then the device is determined to be a powered device in compliance with the PoE standard. The PoE Standard also specifies that the voltage provided by the power source equipment should be between about 2.8 V and 10 V, the current should be less than about 5 mA, and the voltage difference of the testing signal should be more than 1 V.
During the inspection, a typical method is for the power source equipment to apply a voltage or current to the particular connecting port, and then to measure the response signal from the target device after a predetermined period. The signature resistance is calculated based on the current/voltage relationship from the two signals. If a current is applied, the current is normally in the range of 150 μA to 400 μA, and the voltage of the connecting port is measured to calculate the value of the signature resistance. In this case, a target device in compliance with the PoE standard will cause the power sourcing device to detect an about 2.8 V to 10 V voltage drop in the connecting port.
Alternatively, if the testing signal is a voltage signal, the voltage is generally between about 2.8V to 10V, and the detected current value shall be between about 87.5 uA to 625 uA.
Based on the result, the power source equipment will decide whether to perform classification on the powered device. From the classification, the power source equipment may provide different power to different the power devices.
U.S. Pat. No. 7,856,561 provides an equipment to detect a legacy device in a PoE system. The power source equipment therein will apply a serial signal, including current I1 and I2, to the target connecting port. Respectively after 160 ms, it will detect the voltage of the connecting port as V1 and V2. Based on the ratio between the voltage difference and current difference, it will determine whether the target device associated with the connecting port is a standard-compliant powered device, or a compatible device. Specifically, it will calculate the value of Rdet=(V1−V2)/(I1−I2). If Rdet is within a certain range, it will determine that the target device is a standard-compliant powered device. If the value of Rdet is less than the predetermined value or is a negative value, then it will determine that the target device is a legacy device. If none of the above conditions has been met, then it will determine that the device is neither a standard-compliant powered device nor a legacy device.
Chinese Patent No. CN101031861 discloses a classification method for the powered devices in a wired data telecommunication network. It applies multiple periodic testing signals to a powered device to generate a multi-group classification result. The classification result is mapped to an index table to produce a code used as the classification code.
In addition to the voltage range requirement, the PoE standard also requires that the capacitance of the powered device shall be 0.15 uF. If the capacitance is above 10 uF, it must refuse to supply power. Under this requirement, the power source equipment must also determine the capacitance of the target device to determine whether it is a powered device that it can supply power to.
U.S. Pat. No. 8,412,961 discloses a circuit and method for detecting a legacy device in a power over Ethernet system. The method can determine whether the target device connecting to the particular connecting port is a powered device in compliance with the PoE standard or a legacy device. The method uses a three-level voltage or current as the testing signal, and based its determination on the detected current or voltage measured in the connecting port. The patent also discloses that the three levels can in fact be two levels, whereas the first level and the third level are the same, i.e., the third level applies the same measurement as the first level does.
The patent also claims to detect the capacitance of the target device. However, capacitance calculation is more complicated than voltage calculation, and it requires CPU resource. In addition, for a device with capacitance that is above the range specified by the PoE Standard, it will take longer to charge and discharge, and the device may not yet reach steady state during voltage measurement. If so, it will cause measurement error. According to the patent, the measurement equipment will need to take multiple samples in order to obtain current/voltage signals that are representative as the steady state signal.
Prior PoE system is limited by its ability to determine the powered device due to the variety of devices connected to the connecting ports. A lot of them can be unknown to the PoE system. These devices have different characteristics that often affect the detection accuracy. For example, the resistance and capacitance characteristics of the target device often affect the response signal. This causes incorrect the sampled voltage/current values to be incorrect, especially when it has yet to reach the steady state.
Furthermore, there is yet to exist an easy method of determining the capacitance of the target device. Although the capacitance and the resistance of the target device may be separately measured, but whether a device is suitable for supplying power to is in fact based on the capacitance-resistance combination. Moreover, even with the capacitance-resistance combination, it is necessary to go through a complicated process to come to a correct determination.